Pencil Graphite Electrocatalytic Sensors Modified by Pyrene Coated Reduced Graphene Oxide Decorated with Molybdenum Disulfide Nanoroses for Hydrazine and 4-Nitrophenol Detection in Real Water Samples

Novel nanostructured platforms based on Pencil Graphite Electrodes (PGEs), modified with pyrene carboxylic acid (PCA) functionalized Reduced Graphene Oxide (rGO), and then decorated by chronoamperometry electrodeposition of MoS2 nanoroses (NRs) (MoS2NRs/PCA-rGO/PGEs) were manufactured for the electrocatalytic detection of hydrazine (N2H4) and 4-nitrophenol, pollutants highly hazardous for environment and human health. The surface morphology and chemistry of the MoS2NRs/PCA-rGO/PGEs were characterized by scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS), assessing the coating of the PCA-rGO/PGEs by dense multilayers of NRs. N2H4 and 4-nitrophenol have been monitored by Differential Pulse Voltammetry (DPV), and the MoS2NRs/PCA-rGO/PGEs electroanalytical properties have been compared to the PGEs, as neat and modified by PCA-rGO. The MoS2NRs/PCA-rGO/PGEs demonstrated a higher electrochemical and electrocatalytic activity, due to their high surface area and conductivity, and very fast heterogeneous electron transfer kinetics at the interphase with the electrolyte. LODs lower than the U.S. EPA recommended concentration values in drinking water, namely 9.3 nM and 13.3 nM, were estimated for N2H4 and 4-nitrophenol, respectively and the MoS2NRs/PCA-rGO/PGEs showed good repeatability, reproducibility, storage stability, and selectivity. The effectiveness of the nanoplatforms for monitoring N2H4 and 4-nitrophenol in tap, river, and wastewater was addressed.


Introduction
Over the past years, the increased environmental contamination by toxic pollutants caused by industrialization, agriculture activities, and urban life, has raised a global concern for their harmful effects on human health and biodiversity, making urgent the need for sustainable development [1][2][3].Globally, approximately 80% of industrial and municipal wastewater is discharged into the environment without any pre-treatment, and this situation has become a crucial concern in less developed countries, where there are no infrastructures for wastewater remediation [4].Contaminations of rivers and wastewaters have constantly exposed people to toxic compounds causing numerous mental and physical dysfunctions, cancer, and weakening of the body's immune system, lowering life expectancy, and resulting, in many cases, in mortality [5].Around 829,000.00people, including 300,000.00 children under five years old, die every year from diseases resulting from lack of hygiene and polluted drinking water [6].
In this frame, the manufacturing of innovative and sustainable solutions, user-friendly and cost-effective, for reliable quality control of the waters, is increasingly in demand.
Hydrazine (N 2 H 4 ) is among the most dangerous pollutants because its toxicity can generate irreversible cell damage, and develop complications such as brain and liver dysfunction, DNA damage, and leukemia.Despite this, N 2 H 4 has numerous uses as a reducing agent, emulsifier, catalyst, antioxidant, corrosion inhibitor, and as a precursor of explosives, dyestuffs, pesticides, herbicides, insecticides, and pharmaceutical derivatives.U.S. Environmental Protection Agency (EPA) has classified it as a potent carcinogen, with a recommended level in drinking water lower than 10 ppb [7].4-nitrophenol (4-NP) is another potential carcinogen and mutagenic agent that causes acute effects such as headache, nausea, drowsiness, cyanosis, and cancer.4-NP is used in industries of synthesis of drugs, leather processing, dye synthesis, and preparation of organo-phosphorus pesticides, such as methyl parathion and ethyl parathion, although it is in the "Priority Pollutant List" of U.S. EPA, with a recommended upper limit in drinking water of 10 ppb [8].
Among the conventional analytical technologies used for monitoring N 2 H 4 and 4-NP, there are gas chromatography/mass spectrometry (GC/MS), atomic absorption spectroscopy (AAS), high-performance liquid chromatography (HPLC), spectrofluorimetry, capillary electrophoresis, and flow injection chemiluminescence [9,10].Such analytical tools are laborious, expensive, require advanced skills for their operation, and are difficult to install for the bulky size of their devices.By contrast, electrochemical sensors offer the advantage of the rapidity of analysis and cost-effectiveness, and their reduced size makes them portable and usable on-site [11].Electrode modification is the strategy used to improve the sensitivity and selectivity of these sensors, reproducibility of the electrode surface behavior, and accelerate the kinetics of the electrochemical reactions of several compounds [12].
Graphene-based nanostructures have found application in this type of sensor for their high electrochemical stability, high electrocatalytic activity, and fast heterogeneous electron transfer kinetics [13].Also, 2D layer-structured transition-metal dichalcogenide nanomaterials, such as molybdenum disulfide (MoS 2 ) semiconductors, have attracted attention for their interesting electric tunable properties, depending on crystalline structure, nanosheet size, and structural surface defects [14].The preparation of hybrid nanocomposites based on graphene derivatives decorated with nanostructured MoS 2 results in materials showing enhanced stability, electron conductivity, heterogeneous electron transfer kinetics, and electrocatalytic activity [15].
Herein, hybrid nanocomposite-modified nanoplatforms formed by pencil graphite electrodes (PGEs) coated by Reduced Graphene Oxide (rGO) sheets, functionalized with 1-pyrene carboxylic acid (PCA), then decorated by chronoamperometry electrodeposition, with a dense layer of MoS 2 nano roses (NRs), have been investigated for the electrocatalytic detection of N 2 H 4 and 4-NP by Differential Pulse Voltammetry (DPV).
1-pyrene carboxylic acid (PCA) has been used to allow liquid phase exfoliation of rGO [16], as it acts as a linker for binding the rGO basal plane by π-π interactions and the MoS 2 NRs by its carboxyl functionalities, and to promote NRs-rGO electron coupling, providing effective merging of functionalities of the two materials [17].
PGE-based sensors have been selected, because are a practical and not expensive sensing technology, rapid, compact, and suited for portable use and for on-site monitoring.
The manufactured MoS 2 NRs/PCA-rGO/PGEs have shown a LOD for N 2 H 4 and 4-NP of 9.3 nM and 13.3 nM, respectively lower than the U.S. EPA recommended concentration in drinking water and comparable with the lowest ones reported [18][19][20][21][22][23], with values of repeatability, reproducibility, storage stability, and selectivity, suited for monitoring the selected hazardous, in tap, river, and wastewater samples.

Decoration of the PCA-rGO/PGEs with MoS 2 NRs and Characterization
The MoS 2 NRs/PCA-rGO/PGEs were manufactured starting from the liquid phase exfoliation [16] of rGO with 1-pyrene carboxylic acid (PCA) (Figure 1A), which binds by π-π interactions the rGO basal plane and the by the carboxyl functionalities the MoS 2 NRs, enabling NRs-rGO electron coupling [17].Then, the PCA-rGO/PGEs were prepared by dipping the PGEs in an ethanol dispersion of PCA-rGO, and subsequently electrodepositing the MoS 2 NRs by chronoamperometry, after dipping in a 5 mM (NH 4 ) 2 MoS 4 precursor solution at pH 7.4 (Figure 1A).
Molecules 2023, 28, x FOR PEER REVIEW 3 of 14 repeatability, reproducibility, storage stability, and selectivity, suited for monitoring the selected hazardous, in tap, river, and wastewater samples.

Decoration of the PCA-rGO/PGEs with MoS2 NRs and Characterization
The MoS2NRs/PCA-rGO/PGEs were manufactured starting from the liquid phase exfoliation [16] of rGO with 1-pyrene carboxylic acid (PCA) (Figure 1A), which binds by ππ interactions the rGO basal plane and the by the carboxyl functionalities the MoS2 NRs, enabling NRs-rGO electron coupling [17].Then, the PCA-rGO/PGEs were prepared by dipping the PGEs in an ethanol dispersion of PCA-rGO, and subsequently electrodepositing the MoS2 NRs by chronoamperometry, after dipping in a 5 mM (NH4)2MoS4 precursor solution at pH 7.4 (Figure 1A).The effectiveness of the chronoamperometry electrodeposition of the MoS2 NRs was investigated by registering the reduction current of the (NH4)2MoS4 precursor solution at different pH and changing the deposition time (Figure S1 of the Supplementary Information) to determine the most suited experimental conditions for achieving the highest reduction current, that was indeed obtained at pH 7.4 (Figure S1A) and after 90 s of electrodeposition (Figure S1B).
Scanning Electron microscopy (SEM), Raman, and X-ray Photoelectron Spectroscopy (XPS) investigation were carried out to study surface morphology and chemistry of the PGEs, as neat, and after deposition of PCA-rGO and electrodeposition of the MoS2 NRs.
The PGEs show the porous surface morphology (Figure S2A) of the graphite texture.After dipping in the PCA-rGO dispersion, the electrodes are coated by sheets-like structures recalling the typical surface morphology of the PCA-rGO sheets (Figure S2B), which appear almost smooth on the surface featuring bright wrinkles ascribed to folded edges and mechanical lattice deformations (Figure 2A).After electrodeposition of (NH4)2MoS4, the PCA-rGO/PGEs surface morphology is characterized by a coating formed by a The effectiveness of the chronoamperometry electrodeposition of the MoS 2 NRs was investigated by registering the reduction current of the (NH 4 ) 2 MoS 4 precursor solution at different pH and changing the deposition time (Figure S1 of the Supplementary Information) to determine the most suited experimental conditions for achieving the highest reduction current, that was indeed obtained at pH 7.4 (Figure S1A) and after 90 s of electrodeposition (Figure S1B).
Scanning Electron microscopy (SEM), Raman, and X-ray Photoelectron Spectroscopy (XPS) investigation were carried out to study surface morphology and chemistry of the PGEs, as neat, and after deposition of PCA-rGO and electrodeposition of the MoS 2 NRs.
The PGEs show the porous surface morphology (Figure S2A) of the graphite texture.After dipping in the PCA-rGO dispersion, the electrodes are coated by sheets-like structures recalling the typical surface morphology of the PCA-rGO sheets (Figure S2B), which appear almost smooth on the surface featuring bright wrinkles ascribed to folded edges and mechanical lattice deformations (Figure 2A).After electrodeposition of (NH 4 ) 2 MoS 4 , the PCA-rGO/PGEs surface morphology is characterized by a coating formed by a multilayer of nanoroses (NRs)-like structures (Figure 2B), generated by the assembling of MoS 2 nanosheets (Figure 2C), as demonstrated elsewhere [24].and 287.32 eV, respectively, and the Mo3p, Mo3d, and S2p components of the MoS2 NRs at 399.53, 232.32 and 161.92 eV [27], respectively (Figure 2E).In the spectra of the MoS2NRs/PCA-rGO/PGEs, the O1s and C1s components are shifted of ca.2.2 eV with respect to the PCA-rGO/PGEs counterparts, which are at 538.12 eV and 289.52 eV, respectively (Figure 2E), attesting a change of the electron densities of the O and C atoms of PCA-RGO, ascribed to the binding, by coordination, of the MoS2 NRs [24], mediated by PCA [25][26][27].The comparison of the Raman spectra of the MoS 2 NRs/PCA-rGO/PGEs and PCA-rGO/PGEs shows the same Raman modes, namely the D peak at 1335 cm −1 , originating from the breathing modes of C-sp 2 atoms in hexagonal rings, and the G peak, which is due to the bond stretching between two C-sp 2 atoms at ca. 1530 cm −1 [23] (Figure 2D).The intensity ratio between the D and G peaks of rGO is almost preserved after NRs electrodeposition (Figure 2D) and is typically used as an indication of the graphitic material structural quality [25], such a result addresses NRs formation with preservation of the rGO structure.These spectra also show peaks at 370 cm −1 and 465 cm −1 attributed to the E 1 2g and A 1g modes of the MoS 2 NRs [26], assessing their effective electrodeposition onto the PCA-rGO/PGEs (Figure 2D).
XPS survey spectra of the MoS 2 NRs/PCA-rGO/PGEs further show the electrode change of chemistry, with the typical O1s and C1s components of PCA-RGO at 540.33 eV and 287.32 eV, respectively, and the Mo3p, Mo3d, and S2p components of the MoS 2 NRs at 399.53, 232.32 and 161.92 eV [27], respectively (Figure 2E).In the spectra of the MoS 2 NRs/PCA-rGO/PGEs, the O1s and C1s components are shifted of ca.2.2 eV with respect to the PCA-rGO/PGEs counterparts, which are at 538.12 eV and 289.52 eV, respectively (Figure 2E), attesting a change of the electron densities of the O and C atoms of PCA-RGO, ascribed to the binding, by coordination, of the MoS 2 NRs [24], mediated by PCA [25][26][27].
The electrodeposition of the MoS 2 NRs onto PCA-rGO was studied by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) (Figure 3) using the Fe[(CN) 6 ] 3−/4− probe.The CV curves recorded at the MoS2NRs/PCA-rGO/PGEs and PCA-rGO/PGEs show a decrease in the anodic and cathodic peak potentials difference (ΔEp) with respect to PGEs, which show the typical quasi-reversible redox peaks of [Fe(CN)6] 3−/4− (Figure 3A).This result shows higher reversibility of the probe at both the modified electrodes, which is accounted for by their higher conductivity and higher electron transfer capability with the electrolyte, as demonstrated by the increase in their K0 with respect to the PGEs (Table 1).In particular, the decrease in ΔEp is higher at the MoS2NRs/PCA-rGO/PGEs than at the PCA-rGO/PGEs, (Figure 3A) for (i) occurrence of MoS2NRs-rGO electron coupling interactions mediated by PCA that are responsible for the increase in the electrode conductivity [17][18][19][20][21][22][23][24][25][26][27][28][29], and (ii) the catalytic activity of the MoS2 NRs that favors the [Fe(CN)6] 3−/4− red/ox reactions at the electrode [15].Besides, PCA increases the electric conductivity of the PCA-rGO/PGEs acting as electrical "glue" among rGO sheets, [30] and bears oxygen-containing moieties undergoing red/ox reactions, [31] that increase K0 of the PCA-rGO/PGEs.Finally, a significant increase in the anodic current is observed at the MoS2NRs/PCA-rGO/PGEs (Figure 3A), likely for their higher Aele (Table 1), assessing their higher electrochemical activity.This evidence is supported also by the Faradaic impedance spectra of Figure 3B, which show, for the MoS2NRs/PCA-rGO/PGEs, a reduction of the semicircle diameter, demonstrating a decrease in Ret (Table 1), which confirms the higher reversibility of the probe at such electrodes.

Electrochemical Detection of N2H4 and 4-NP at the MoS2NRs/PCA-rGO/PGEs
The manufactured MoS2NRs/PCA-rGO/PGEs were studied for the detection of N2H4 and 4-NP, respectively, by cyclic voltammetry (CV) in a three-electrode cell (Figure 1B).At first, the electrode response to the selected analytes was investigated in the range of pH of the (NH4)2MoS4 precursor solution and of the analyte solutions between 3 and 9.5.The results show that the higher electrocatalytic activity and sensitivity of the MoS2NRs/PCA-rGO/PGEs were achieved with (NH4)2MoS4 (Figure S3A) and analytes solutions at pH 7.4 The CV curves recorded at the MoS 2 NRs/PCA-rGO/PGEs and PCA-rGO/PGEs show a decrease in the anodic and cathodic peak potentials difference (∆Ep) with respect to PGEs, which show the typical quasi-reversible redox peaks of [Fe(CN) 6 ] 3−/4− (Figure 3A).This result shows higher reversibility of the probe at both the modified electrodes, which is accounted for by their higher conductivity and higher electron transfer capability with the electrolyte, as demonstrated by the increase in their K 0 with respect to the PGEs (Table 1).In particular, the decrease in ∆Ep is higher at the MoS 2 NRs/PCA-rGO/PGEs than at the PCA-rGO/PGEs, (Figure 3A) for (i) occurrence of MoS 2 NRs-rGO electron coupling interactions mediated by PCA that are responsible for the increase in the electrode conductivity [17][18][19][20][21][22][23][24][25][26][27][28][29], and (ii) the catalytic activity of the MoS 2 NRs that favors the [Fe(CN) 6 ] 3−/4− red/ox reactions at the electrode [15].Besides, PCA increases the electric conductivity of the PCA-rGO/PGEs acting as electrical "glue" among rGO sheets, [30] and bears oxygen-containing moieties undergoing red/ox reactions, [31] that increase K 0 of the PCA-rGO/PGEs.Finally, a significant increase in the anodic current is observed at the MoS 2 NRs/PCA-rGO/PGEs (Figure 3A), likely for their higher A ele (Table 1), assessing their higher electrochemical activity.This evidence is supported also by the Faradaic impedance spectra of Figure 3B, which show, for the MoS 2 NRs/PCA-rGO/PGEs, a reduction of the semicircle diameter, demonstrating a decrease in R et (Table 1), which confirms the higher reversibility of the probe at such electrodes.The manufactured MoS 2 NRs/PCA-rGO/PGEs were studied for the detection of N 2 H 4 and 4-NP, respectively, by cyclic voltammetry (CV) in a three-electrode cell (Figure 1B).At first, the electrode response to the selected analytes was investigated in the range of pH of the (NH 4 ) 2 MoS 4 precursor solution and of the analyte solutions between 3 and 9.5.The results show that the higher electrocatalytic activity and sensitivity of the MoS 2 NRs/PCA-rGO/PGEs were achieved with (NH 4 ) 2 MoS 4 (Figure S3A) and analytes solutions at pH 7.4 (Figure S3B).

Electroanalytical Investigation of the
The CV curves collected at the PGEs and PCA-rGO/PGEs in the presence of N 2 H 4 do not present any cathodic peak.Conversely, the MoS 2 NRs/PCA-rGO/PGEs show an intense peak at 0.13 V (vs.Ag/AgCl, saturated KCl) (Figure 4A) accounted for by the N 2 H 4 oxidation catalyzed by MoS 2 NRs [32], that leads to formation of N 2 H 4 + then oxidized to N 2 [33].The lack of a peak in the cathodic sweep of N 2 H 4 addresses the irreversibility of the oxidation process (Figure 4A) [33].
The CV curves collected at the PGEs and PCA-rGO/PGEs in the presence of N2H4 do not present any cathodic peak.Conversely, the MoS2NRs/PCA-rGO/PGEs show an intense peak at 0.13 V (vs.Ag/AgCl, saturated KCl) (Figure 4A) accounted for by the N2H4 oxidation catalyzed by MoS2 NRs [32], that leads to formation of N2H4 + then oxidized to N2 [33].The lack of a peak in the cathodic sweep of N2H4 addresses the irreversibility of the oxidation process (Figure 4A) [33].
Besides, the increase in the current intensity at the PCA-rGO/PGEs and MoS 2 NRs/PCA-rGO/PGEs is accounted for by the enhancement of their A ele (Table 1).
Finally, in Figure 4B, a peak between −0.1-0.3V, due to the oxidation of 4-quinoimine, the reduction product of 4-aminophenol [34], is evident.Such a peak is more intense and shifts toward lower potential values at the MoS 2 NRs/PCA-rGO/PGEs (Figure 4B) for the electrocatalytic properties of the NRs [15].
The trend of the current intensity against the square root of scan rate (v 1/2 ) of N 2 H 4 and 4-NP was collected to study the charge transport across the MoS 2 NRs/PCA-rGO/PGEs and mass transfer regime (Figure 4C,D).The oxidation current of N 2 H 4 and the reduction current of 4-NP can be fitted by linear regression, with a correlation coefficient of 0.99, increasing linearly with the increase in v 1/2 (Figure 4C,D).These results assess the occurrence of diffusion-controlled electron transfers.
The results present a linear relationship of I cat /I 0 vs. t 1/2 , and K cat values of 7.1 mM −1 s −1 for N 2 H 4 and 6.2 mM −1 s −1 for 4-NP, attesting to the high electrocatalytic activity of the MoS 2 NRs/PCA-rGO/PGEs.the reduction product of 4-aminophenol [34], is evident.Such a peak is more intense and shifts toward lower potential values at the MoS2NRs/PCA-rGO/PGEs (Figure 4B) for the electrocatalytic properties of the NRs [15].
The trend of the current intensity against the square root of scan rate (v 1/2 ) of N2H4 and 4-NP was collected to study the charge transport across the MoS2NRs/PCA-rGO/PGEs and mass transfer regime (Figure 4C,D).The oxidation current of N2H4 and the reduction current of 4-NP can be fitted by linear regression, with a correlation coefficient of 0.99, increasing linearly with the increase in v 1/2 (Figure 4C,D).These results assess the occurrence of diffusion-controlled electron transfers.

Determination of LOD, Repeatability, Reproducibility, and Storage Stability of
MoS2NRs/PCA-rGO/PGEs and Interference Effects Differential Pulse Voltammetry (DPV) investigation was carried out at the MoS2 NRs/PCA-rGO/PGEs in N2H4 and 4-NP standard solutions, respectively, in the concentration range of 25 µM-1200 µM, to evaluate their electrocatalytic properties and collect calibration curves (Figure 5).
The DPV curves present an increase in the electrocatalytic current with the enhancement of the analyte concentration, showing a linear relationship of type (y = (a ± b)x + c ± d) with a correlation coefficient of r 2 = x.Besides, both the calibration plots have two different slopes (Figure 5), demonstrating two different electrocatalytic kinetic processes, that depend on the analyte concentration, and are likely ascribed to a change of the electrode surface chemistry induced by the red/ox reactions [35].At low concentrations, the electrocatalytic processes evolve by analyte adsorption at the electrode surface active sites, providing a high sensitivity.At higher concentrations, the surface sites are partially saturated, and hence, the activation step of the analyte in the red/ox reaction is slowed down, becoming the rate-determining step that decreases sensitivity [35].The DPV curves present an increase in the electrocatalytic current with the enhancement of the analyte concentration, showing a linear relationship of type (y = (a ± b)x + c ± d) with a correlation coefficient of r 2 = x.Besides, both the calibration plots have two different slopes (Figure 5), demonstrating two different electrocatalytic kinetic processes, that depend on the analyte concentration, and are likely ascribed to a change of the electrode surface chemistry induced by the red/ox reactions [35].At low concentrations, the electrocatalytic processes evolve by analyte adsorption at the electrode surface active sites, providing a high sensitivity.At higher concentrations, the surface sites are partially saturated, and hence, the activation step of the analyte in the red/ox reaction is slowed down, becoming the rate-determining step that decreases sensitivity [35].
Repeatability was investigated by measuring the electrocatalytic current of N 2 H 4 and 4-NP nine times in one day (Figure S4) at the same and % RSD of 3.3 and 3.6, respectively were estimated (Table 2), as shown by Figures S5A and S6A.Reproducibility was assessed using nine hybrid platforms (Figure S7), and % RSD of 3.4 and 3.7 (Table 2), respectively were found as evidenced by Figures S5B and S6B.The storage stability was determined over a period of one month, monitoring the electrocatalytic currents of nine MoS 2 NRs/PCA-rGO/PGEs stored at 4 • C, every week (Figure S8), revealing almost stable values (Table 2), as shown by Figures S5C and S6C.
Matrix components can detrimentally affect the LOD, LOQ, repeatability, and reproducibility of the measurements.For this reason, the selectivity of the MoS 2 NRs/PCA-rGO/PGEs was tested in the presence of the typical interferents of N 2 H 4 and 4-NP.Citric acid, uric acid, ethanol, and glucose were chosen to test the selectivity towards N 2 H 4 , whilst catechol, hydroquinone, and 2,4-dinitrobenzene for 4-NP (Figure S9).DPV curves of the MoS 2 NRs/PCA-rGO/PGEs were recorded in 0.1 M PBS solutions (pH 7.4) and 0.8 mM in N 2 H 4 and 4-NP, respectively, separately spiked with 100-fold more concentrated interfering species.The results show that, although added at a 100-fold higher concentration, the interferent species do not significantly affect the current intensities (Figure S9) and show a % RSD of 3.7% for both the analytes, demonstrating a high electrode selectivity.

Quantification of N 2 H 4 and 4-NP in Real Samples
The effectiveness of the MoS 2 NRs/PCA-rGO/PGEs in the determination of N 2 H 4 and 4-NP in river, tap, and wastewater, was studied by chronoamperometry (Figure S10), performing the analyses by the standard addition method, as described in the experimental section, and comparing the results with those obtained from HPLC analyses (Table 3).
As shown in Table 3, the achieved recovery rates demonstrate the reliability of the MoS 2 NRs/PCA-rGO/PGEs in the detection of the selected analytes in real water samples.
Raman spectra were collected by a LabRAM HR Evolution spectrophotometer from HORIBA, equipped with a 100× microscope objective lens and a continuous excitation laser diode at 532 nm.
X-ray Photoelectron Spectroscopy (XPS, Kratos Axis Ultra) was performed by a monochromatic Al Ka source (at 1486.58 eV), operating with a spot size of 200 µm, at a take-off angle of 70 • .Survey (0-1000 eV) spectra were collected at a pass energy of 160 eV.Charge correction of the spectra was performed considering the sp 2 carbon component of the C1s spectrum as an internal reference (Binding Energy, BE = 30 eV).
Scanning Electron Microscopy (SEM) images were recorded by a Zeiss Sigma microscope, equipped with both an in-lens secondary electron and an INCA Energy Dispersive Spectroscopy (EDS) detector.Samples were fixed onto stainless-steel holders by using carbon tape.
Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV), Chronoamperometry, and Electrochemical Impedance Spectroscopy (EIS) measurements were performed by a Metrohm Autolab PGSTAT 302n electrochemical workstation (Herisau, Switzerland), equipped with the Nova ® v1.11 software and a three-electrode cell, where the pencil graphite electrode (PGE), a platinum wire and an Ag/AgCl (3 M KCl) electrode are the working, counter, and reference electrodes, respectively (Figure 1).The electrical connection between the electrochemical workstation and the PGEs was settled welding a copper wire onto the metallic holder of the graphite pencil.

Exfoliation and Functionalization of Reduced Graphene Oxide (rGO) with 1-Pyrene Carboxylic Acid (PCA)
PCA-rGO was prepared by exfoliating and functionalizing commercial rGO with PCA following a reported procedure with minor modifications [36], stirring and sonicating a mixture of PCA and rGO powders prepared in a 17:1 w/w in n-methyl-2-pyrrolidone (NMP), in an ice-cooled bath.Centrifugation cycles (9000 rpm for 20 min) and re-dispersion in methanol were carried out to remove PCA in excess.The purified PCA-rGO complex is formed of flakes of single and few-layer graphene and multi-layer graphene [16,17].

Modification of PGEs with PCA-rGO and Decoration with MoS 2 Nanoroses
PGEs were polished with a weighing paper to achieve an almost flat surface and then were sonicated in a 1 M H 2 SO 4 solution, for 2 min, to graft oxygen-based groups, leading to a significant increase in the PGE electrochemical reactivity [37].
The PCA-rGO modified PGEs (PCA-rGO/PGEs) were obtained by soaking 10 mm of a 0.7 mm 2H graphitic pencil, into a 2.5 mg mL −1 PCA-rGO dispersion in ethanol for 30 min.In this step, PCA-rGO binds the H 2 SO 4 -treated graphitic electrode by π-π stacking forces and hydrogen bond interactions.

Electrochemical Investigation of the MoS 2 NRs/PCA-rGO/PGEs
The functionalization of the PGEs with PCA-rGO and decoration with MoS 2 NRs were studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), in a 0.01 M PBS buffer solution added with 0.1 M KCl and 5 mM Fe[(CN) 6 ] 3−/4− at pH 7.4.CV scans were collected at the 50 mV s −1 scan rate.
Faradaic impedance spectra were reported as Nyquist plots, and the collected data were treated by Randles equivalent circuits by the Nova ® v1.11 software to estimate charge transfer resistance (R et ).
Electroactive surface area (A ele ) was calculated by the Randles-Sevcik equation for a quasi-reversible system, as: I ap = (2.69 × 10 5 ) A ele × C × D 1/2 × n 3/2 × v 1/2  (1) where I ap is the anodic peak current, n the number of electrons transferred, D the [Fe(CN) 6 ] 4− diffusion coefficient equal to 6.5 × 10 −6 cm 2 s −1 , v the potential scan rate (V s −1 ) and C the [Fe(CN) 6 ] 4− concentration (mol cm −3 ).The heterogeneous electron transfer rate constant (k 0 ) was determined as: where R is the universal gas constant and F is the Faraday constant.Electrocatalytic rate constants (K cat ) were calculated by chronoamperometry in 0.1 M PBS buffer solutions at pH 7.4, 1 mM in N 2 H 4 and 4-NP, respectively, by using the Cottrell equation as [38]: where I cat and I 0 are the currents collected with and without the analyte, respectively at the concentration C, K cat is the electrocatalytic rate constant, and t is the measurement time.

Figure 1 .
Figure 1.Scheme of (A) exfoliation and functionalization of rGO with PCA, deposition of rGO onto PGEs and electrochemical deposition of MoS2 NRs onto PCA-rGO/PGEs, and (B) electrochemical analytes detection.

Figure 1 .
Figure 1.Scheme of (A) exfoliation and functionalization of rGO with PCA, deposition of rGO onto PGEs and electrochemical deposition of MoS 2 NRs onto PCA-rGO/PGEs, and (B) electrochemical analytes detection.

N 2 H
4 and 4-NP were monitored by differential pulse voltammetry (DPV) dipping the PGEs for 10 mm into a 0.1 M PBS buffer solution at pH 7.4, added by N 2 H 4 and 4-NP, in the concentration range of 20-1200 µM, respectively, with modulation time of 0.05 s, interval

Table 3 .
Determination of N 2 H 4 and 4-NP in tap, river, and wastewater.